WSP02004

• File Number: 8200.760
• Description: Yampa River General
• State: CO
• Basin: Yampa/White
• Water Division: 6
• Date: 11/28/1992
• Author: Unknown
• Title: Yampa River Feasibility Study - Model Sensitivity to the Size of the Juniper Contemplated Draft September 28 1992
• Water Supply Pro - Doc Type: Data

l'Hl1.474

.

.

\

DIR

sc

TO:
FROM:
SUBJECT:
DATE:

Ray Tenney
Chuck Brendecke, Ed Armbruster
Model sensitivity to the size of the Juniper contemplated draft.
September 28, 1992

ASC

In: _,==~.1

Following a meeting of the Technical Steering Group (TSG) on February 13, 1992,
several sensitivity runs were made with the Yampa model to evaluate the effects of the size of
the Juniper contemplated draft, as converted to an instream flow at Juniper Canyon, on basin
demands and reservoir contents. This effort was made after sollie discussions on the
appropriateness of using the contemplated draft defined in a 1989 W.W, Wheeler study in light
of the fact that the Hydrosphere model incorporated several different assumptions with regards
to basin demands and reservoir operations, The Wheeler contemplated draft had been
developed based on the results of the VSFWS HYDROSS model and the "environmental
baseline" flows predicted for the Yampa River at Maybell, The sensitivity test involved
increasing and decreasing the Wheeler contemplated draft by 5 % and examining the differences
in modeled shortages and reservoir contents, The results indicated that there was very little
effect and at the direction of the TSG, it was decided to stay with the Wheeler numbers for the
current modeling effort,

In May of 1992, several modifications were made to the Yampa River model to more
accurately represent reservoir operations, Included was an adjustment to the routing of
reservoir spills that had the potential to understate the capacity of reservoir outlet works,

We have recently re-run the contemplated draft sensitivity tests with the modified model.
The results of these tests are attached with this memo. These tests were similar to the original
sensitivity tests and examined the effects of a 5 % change in the contemplated draft, The
original contemplated draft over the period 1930 to 1982 averaged 862,011 af per year. The
sensitivity model runs were made using drafts of 818,911 af and 905,112 afper year, The
results included here include end-of-month reservoir contents at Elkhead Reservoir and
shortages to modeled future demands.

Results of these tests indicate some small changes to demand shortages and reservoir
contents with changes to the contemplated draft, The model run using a higher contemplated
draft results in Elkhead Reservoir operating an average of 1,670 af lower each month (Tables
1, 3 and 4), Future demands were shorted an average of 245 af per year more than under the
Wheeler contemplated draft model run, or approximately 6 percent (Tables 2, 5 and 6).

When the contemplated draft was reduced by 5 %, modeled Elkhead Reservoir contents
were an average of 1,590 af bigher (Tables 1, 7 and 8), Modeled shortages to future demands
were reduced by an average of 358 af per year, or approximately 8 percent (Tables 2, 9 and
10),

It appears that some of the counter-intuitive results of our earlier sensitivity analysis
were artifacts of the treatment of reservoir spills, With the modification of this aspect of the
model, the sensitivity of storage and shortages to changes in the contemplated draft is as
expected.

Hydrosphere Resource Consultants 1002 Walnut Suite 200 Boulder, Colorado 80302

~,\PIlQJECTSI'U\r"'lof\MDOe~\''IOTO\SC.'l_J\:lC\TeST\Me'''054.ROTClle'''TEO' S.,YEM8eR29. 'in "'~,o,M PIlFNTeo: SH<o""''' '" t...., .... .